In recent years, particulates, contained in exhaust gases discharged from internal combustion engines of vehicles such as buses and trucks and construction machines, have raised serious problems since those particulates are harmful to the environment and the human body.
For this reason, there have been proposed various ceramics filters which allow exhaust gases to pass through porous ceramics to collect particulates in the exhaust gases, so that the exhaust gases can be purified.
Normally, the ceramic filter of this type has a structure in that a number of through holes are arranged side by side in one direction and partition wall that separate the through holes from each other is allowed to function as filters.
In other words, each of the through holes formed in the ceramic filter is sealed with a plug at either of ends of its exhaust-gas inlet side and outlet side, so that exhaust gases that have entered one through hole are discharged from another through hole after having always passed through each part of the partition wall that separates the through holes; thus, when exhaust gases are made to pass through the partition wall, particulates are captured by the partition wall so that the exhaust gases are purified.
As such a purifying process for exhaust gases progresses, particulates are gradually accumulated on the partition wall that separates the through holes of the ceramic filter to cause clogging and the subsequent interruption in gas permeability. For this reason, the above-mentioned ceramic filter needs to be subjected to a recycling process regularly by burning and removing the particulates that cause clogging using heating means such as a heater.
In such a recycling process, however, it is difficult to evenly heat the ceramic filter, thus the heat is locally generated due to the burning of the particulates to cause a large thermal stress. Moreover, even during normal operations, an uneven temperature distribution occurs inside the ceramic filter due to a thermal impact or the like derived from an abrupt temperature change in the exhaust gases, resulting in a thermal stress.
Consequently, in the case where the ceramic filter is constituted by a single ceramic member, cracks tend to be generated to cause a serious problem in collecting the particulates.
Moreover, in an attempt to produce a large-size ceramic filter, since firing shrinkage becomes greater upon firing, it becomes difficult to control the shape.
For this reason, there has been proposed a honeycomb filter having the following structure: a ceramic filter is divided into a plurality of porous ceramic members, with a number of through holes formed therein, and the porous ceramic members are combined with one another through adhesive layers.
With the honeycomb filter of this type, it becomes possible to reduce a thermal stress that is exerted on the honeycomb filter during recycling processes and operations, and also to freely adjust the size thereof by increasing or reducing the number of the porous ceramic members.
Incidentally, in the conventional honeycomb filter having this structure, it has been considered that it is preferable to set the thermal expansion coefficient of the porous ceramic member and the thermal expansion coefficient of the adhesive layer in the same level.
The reason for this is explained as follows: actually, the above-mentioned honeycomb filter is used in a wide temperature range, for example, from 10 to 800° C., and when the thermal expansion coefficient of the porous ceramics member is different from the thermal expansion coefficient of the adhesive layer, cracks tend to be generated in the porous ceramic member and the adhesive layer due to the difference in the coefficients of these members.
However, in the case where the thermal expansion coefficient of the porous ceramic member and the thermal expansion coefficient of the adhesive layer are made completely identical to each other, this case is the same as the case using a single ceramic member. Therefore, when particulates are burned locally in the honeycomb filter, that is, when a local temperature change occurs therein, due to: uneven amounts of accumulated particulates; uneven amounts of catalyst in the case of allowing the honeycomb filter to support the catalyst; and unevenness of applied heat given by a heater, exhaust gases and the like, a great thermal stress is generated between the portion having this local temperature change and the other portions, with the result that cracks tend to be generated in the porous ceramic member and the adhesive layer.
Whereas, related to this problem, JP Kokai 2001-190916 has disclosed a honeycomb filter in which Young's modulus of the adhesive layer is set to 20% or less of Young's modulus of the porous ceramic member and a honeycomb filter in which the material strength of the adhesive layer is made lower than the material strength of the porous ceramic member. Here, in the honeycomb filter in which Young's modulus of the adhesive layer is made lower than Young's modulus of the porous ceramic member, the adhesive layer having a comparatively low Young's modulus tends to be extended more easily than the porous ceramic member upon application of the same force onto the adhesive layer and the porous ceramic member, thereby making it possible to alleviate the thermal stress to be imposed on the porous ceramic member.
However, in the case of the honeycomb filter in which Young's modulus of the adhesive layer is set to 20% or less of Young's modulus of the porous ceramic member and the honeycomb filter in which the material strength of the adhesive layer is made lower than the material strength of the porous ceramic member, the kinds of the materials of the adhesive layer and the porous ceramic member to be used are limited. Moreover, even in the case of the above-mentioned honeycomb filters, when the coefficient of the thermal expansion of the porous ceramic member and the coefficient of the thermal expansion of the adhesive layer are equivalent to each other, in the case of occurrence of a local temperature change, it was not possible to sufficiently alleviate a great thermal stress, resulting in the problem of the generation of cracks in the porous ceramic member and the adhesive layer.